1. Field of the Invention
This invention relates to an admittance meter for monitoring a medium in a fill level sensor.
2. Description of the Prior Art
The invention relates to an admittance meter for monitoring a medium for a fill level sensor, having at least one electrical alternating signal source, a diode ring operated as a synchronous rectifier with at least four diodes connected in series and in the same direction in succession, a measuring sensor, and an evaluation unit, in which the diode ring can be subjected to an alternating signal is via a first feed point and a second feed point, and the first feed point and the second feed point are each connected via two series-connected diodes of the diode ring, the measuring sensor is connected to the first feed point, and the evaluation unit is connected to a first measurement point of the diode ring and/or to a second measurement point of the diode ring, and in which the first measurement point and the second measurement point are each connected via one diode of the diode ring to the first feed point and the second feed point, respectively.
For many years, admittance meters for fill level sensors have been known in which the measuring principle is based on the fact that the variable fill level of a defined medium in a volume to be monitored affects the measuring sensor employed, and more precisely varies the admittance that is detectable by the measuring sensor. Usually, the measuring sensor is opened electrically into the volume to be monitored or in other words when a capacitive measurement principle is employed has an active, triggered electrode, and an electrical stray field from the active electrode of the measuring sensor extends into the surroundings. The capacitance of the capacitor formed by the active electrode of the measuring sensor and the variable surroundings is then dependent on the fill level to be detected of the medium, just as it is dependent on the electrical material properties of the medium (such as the dielectric constant).
When the measuring sensor of the alternating signal source is subjected to an alternating signal, such an alternating voltage, then it is readily understandable that the measuring current—in the case of the capacitive measuring sensor, the charging and discharging current of the measuring capacitor—is a function of the variable admittance of the measuring sensor, namely the admittance that is dependent on the fill level of the medium. By measuring the current established, a conclusion can be drawn about the admittance of the measuring sensor and thus in turn about the fill level or a change in the fill level but also about the conductivity and the dielectric constant of the medium that is present.
From German Patent Disclosure DE 43 22 867 A1, an admittance meter is known which is not used in conjunction with a fill level sensor but rather serves the purpose of contactless detection of gear wheels of a gear wheel in order to ascertain the position and rotation of the gear wheel. Here, the measuring sensor comprises an active electrode of a capacitor, the electrode being subjected to an alternating signal from the alternating signal source, and the counterelectrode is formed by the gear wheels of the gear wheel. The measuring sensor thus formed is connected to a diode ring operated as a peak-value rectifier, in that the alternating signal source subjects not only the measuring sensor but simultaneously a first feed point and a second feed point of the diode ring to an alternating signal. At a measurement point of the diode ring, a charging or discharging current that is due to the charging and discharging of the capacitors is then picked up, and the modulation frequency of an output signal, furnished by the evaluation unit, serves as a measure for the rpm of the gear wheel. The disposition is advantageous because, as a result of the symmetry of the diode ring operated as a peak value rectifier, the effects of temperature drift of the conducting-state diode voltages cancel one another out. This measuring circuit is suitable for detecting a rapidly varying admittance of the measuring sensor but is not suitable for detecting an only slowly varying admittance of the measuring sensor, of the kind that is the case in many fill level measuring tasks. In fill level measuring tasks, it is therefore—as described at the outset—the amplitude of a measuring signal that is evaluated, not its frequency—which is dependent on the variation of the test subject.
In detail, there are now various possibilities for embodying and refining the admittance meter of the invention. To that end, reference is made to the claims and to the description of preferred exemplary embodiments in conjunction with the drawings. In the drawings: